In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives. For example, one common attribute imparted to tissue sheets through the use of chemical additives is softness, particularly topical or surface softness.
For instance, in some applications, tissue products are treated with polysiloxanes in order to increase the softness of the tissue.
In some applications, tissue products may be treated with other beneficial agents as well. For example, in addition to softening agents such as polysiloxane lotions, other desirable agents may be added to a tissue in order to provide a benefit to the user. For example, vitamins, plant extracts, medications, antimicrobial compounds, and the like may also be added to the web in order to transfer the desired agent to the consumer upon use.
In the papermaking industry, various manufacturing techniques have been specifically designed to produce paper products which consumers find appealing. Manufacturers have employed various methods to apply chemical additives, such as silicone compositions and other beneficial agents, to the surface of a tissue web. Currently, one method of applying chemicals to the surface of a tissue web is the rotogravure printing process. A rotogravure printing process utilizes printing rollers to transfer chemicals onto a substrate. Chemicals that are applied to webs using the rotogravure printing process typically require the addition of water, in combination with, surfactants, in order to prepare an emulsion capable of being applied onto the substrate using conventional technologies. Such additions are not only costly but also increase wet-out time, drying time, and add process complexity.
A similar method to rotogravure printing is also known in the art. In this method the polysiloxane emulsion is applied to a heated transfer roll to remove some of the solvent (water). The concentrated silicone emulsion is then transferred from the heated transfer roll to the surface of the tissue. While this process may provide some benefits from the drying time required by the conventional rotogravure process it still requires the use of dilute solutions/emulsions containing surfactants and therefore does not address the issues of additional chemicals, increased wet out times and process complexity. Additionally, both the rotogravure and transfer roll process require the tissue to be subjected to Z-directional compressive forces which in combination with the water, surfactants and other diluents present tend to reduce the bulk of the finished product. In addition, these Z-directional compressive forces tend to drive the chemicals into the bulk of the tissue whereby the chemical can penetrate a significant distance into the Z-direction of the sheet. As the softening agents applied in this manner are intended to improve the surface feel, the chemical that penetrates in the Z-direction of the sheet is not effective and hence more chemistry is required than if it were all retained on the tissue surface.
Another method of applying chemical additives to the surface of a tissue web is spray atomization. Spray atomization is the process of combining a chemical with a pressurized gas to form small droplets that are directed onto a substrate, such as paper. One problem posed with atomization processes is that manufacturers often find it difficult to control the amount of chemical that is applied to a paper ply. Thus, a frequent problem with spray atomization techniques is that a large amount of over-spray is generated, which undesirably builds upon machinery as well as the surfaces of equipment and products in the vicinity of the spray atomizer. Furthermore, over-spray wastes the chemical being applied, and comprises a generally inefficient method of applying additives to a tissue web.
In addition, many spray atomization devices produce a wide spectrum of droplet diameters. The variability in droplet size makes it difficult to control the amount of chemical additive that is applied to the product. Further, lack of control over the spray atomization technique also affects the uniformity of application to the tissue web.
In view of the above, a need exists in the industry for improving the method for application of chemical additives to the surface of a paper web. Further, a need also exists for tissue products with improved properties due to the manner in which a chemical additive is applied to the product. For example, it is believed that controlled surface application of a softening agent, such as a polysiloxane, may lead to the development of a tissue product having improved surface properties while lowering the levels of the chemical additive needed for a given level of performance.